Near infrared (NIR) illumination is increasingly popular for enhancing the performance and utility of imaging sensors in automotive, mobile and consumer applications. The image sensors are used for object detection, driver monitoring, gesture recognition and other similar user interface functions, with significant use of computational image processing. In addition to adding low light and nighttime capabilities, the illumination can be used to highlight regions of interest and enable filtering out the ambient lighted background for the benefit of image processing algorithms. A major complication for these image processing applications is the modest dynamic range of current electronic image sensors. Subjects or areas of interest captured by the electronic image sensors are often too bright, saturating the detector so that detail is not visible. In other cases, subjects or areas of interest may be too dark, also limiting useful image detail, unless the gain or shutter duration for the imaging system is adjusted so that the brightly illuminated or highly reflective regions are again saturated.
Most current NIR illumination systems are based on light-emitting diodes or LEDs. LEDs have the advantage of low cost and freedom from speckle or coherence noise, which can seriously complicate image processing. The disadvantages of LEDs in this role include the very broad emission profile that is difficult to concentrate to a smaller field and limited optical conversion efficiency at higher powers. See Overton, G. “High-power VCSELs rule IR illumination,” Laser Focus World, Aug., 29-30, (2013). LEDs also have a very broad spectral output, which complicates filtering out solar background and means that some of the light can be visible to the subjects being illuminated, which can be distracting. Conventional laser diode sources can be used for illumination with narrow spectral emission, well defined beams and higher efficiency. However, a single laser source with sufficient power for illuminating the image field will have significant “speckle” or coherence noise from mutual interference of the beam with its own scattered light. In addition, the point source characteristics of single laser sources result in low eye-safe exposure levels.